1. Field of the Invention
The present invention relates to optical devices that are capable of generating diffraction patterns with the central spot size smaller than the diffraction limit or superresolution, for application in optical systems that can make use of this effect such as imaging, confocal scanning microscopy, optical data storage, high-resolution laser printing and laser pattern generation. More particularly, the invention is primarily concerned with phase-only elements, where the phase transmittance varies across the diameter of the element while the amplitude transmission is kept to one, although the amplitude can be varied to provide more design flexibility. These element filters can be fabricated by means of a variety of techniques including diffractive optics technology, holographic methods, thin film deposition, and as gradient-index elements.
2. Related Background Art
It is known that well-corrected optical systems are able to focus most of an incident beam of light into a small region called the beam spot surrounded by a series of low-intensity rings or sidelobes. The size of the beam spot is ultimately limited by the effects of diffraction which determines the maximum resolution achievable in some specific circumstance. For several applications, it is of great interest that the size of the central spot be reduced without affecting too much the behavior of the nearby sidelobes. This effect is known as superresolution. Several applications can benefit from superresolution effects. In scanning confocal microscopy, superresolution techniques increase the resolution with which some object is scanned, resulting in images of higher contrast. In optical data storage, a superresolved beam spot can be used to increase the density of information that can be recorded in an optical disk. Also, it can read information encoded in small pits (bits of information) along a track. In laser printing, a superresolution device increase the density of points that can be printed, in comparison to conventional printing systems.
Prior art involves the use of an obscured aperture to produce a superresolved image (C. J. R. Sheppard, Optik 48 (1977) 329). In another technique, the superresolution device is divided in a typically small number of rotationally symmetric annular sections and the radial positions of the annular sections are varied while the phase transmittance alternately varies among the values 0 and .pi.+2.pi.q, where q is an integer number (J. E. Wilkins, J. Opt. Soc. Am. 40 (1950) 222; Z. S. Hegedus and V. Sarafis, J. Opt. Soc. Am. A 3 (1986) 1892; U.S. Pat. No. 5,349,592 issued to Ando on Sep. 20, 1994). Other methods involving continuous variations of the amplitude transmission function of the superresolution device can also be considered but they offer little advantages over an annular device (I. J. Cox, C. J. R. Sheppard, and T. Wilson, J. Opt. Soc. Am., 72 (1982) 1287).
There are several difficulties associated with the methods previously mentioned. In most applications that can employ superresolution devices, the goal is to reduce the spot size of the central core of the diffraction pattern. However, it is well-known that as the spot size is reduced, the maximum intensity of the central core relative to the diffraction limited spot (Strehl ratio) falls very rapidly. Simultaneously, the relative intensity of the subsequent sidelobes to the central core intensity also increases very rapidly. These effects are extremely undesirable. In imaging, the very low Strehl ratio and high sidelobes causes an effective loss of resolution, since the eye will detect primarily the diffraction rings. In optical disk systems, the low Strehl ratio may not possess enough energy for recording and high sidelobes may cause the appearance of undesired pits. Also in readout, the sidelobes must be kept to acceptable levels in order to avoid reading errors. In scanning confocal microscopy, low Strehl ratio is tolerable to a certain point but high sidelobes can reduce dramatically the usable field of view. Printing systems also require a high Strehl ratio to guarantee that a substrate will be sensitized and low sidelobe intensity to prevent spurious marks from being recorded. Specifically referring to the current methods to achieve superresolution, the method of using obscurations or controlling the amplitude transmittance necessarily causes a reduction of central core intensity. The device with a .pi.+2.pi.q phase shift avoids the absorption of incident light and usually yields small spot sizes but presents strong sidelobe effects. Furthermore the number of available design variables is very limited, being basically restricted to the radial positions of each annular zone. We have found that by properly manipulating the zone boundaries, the phase and amplitude transmission, and the shape of the phase function, it is possible to obtain diffraction patterns of high resolution. The purpose of the present invention is to provide a superresolution mask design that offers a large number of design variables, better or comparable performance than previous methods, high Strehl ratio, proper control of sidelobe effects, and a wide variety of applications.